Production of Synthetic Fuels and Chemicals from Biomass - Biomass Gasification, Pyrolysis and Torrefaction - page 393

Promoters could greatly enhance the performance of catalysts. Potassium
is a good promoter for iron. Copper is also a good promoter for iron catalysts
(Spath and Dayton, 2003), but a Co catalyst is less sensitive to promoters.
Sulfur is a major poisoning element for both Fe and Co catalysts. Thus
coal-based FTS needs an extensive sulfur removal process, while green bio-
mass, being free from sulfur, enjoys a distinct advantage in this respect.
Because a cobalt catalyst is more sensitive to poisoning from sulfur, it is not
used in coal gasified syngas. In that respect, an iron catalyst is better suited
for use with coal-derived syngas.
11.4.2.5 Reactors
Four main types of reactors have been used in FTS ( Figure 11.6 ). They are:
1. Fixed-bed reactor ( Figure 11.6A )
2. Slurry-bed reactor ( Figure 11.6B )
3. Bubbling fluidized-bed (BFB) reactor ( Figure 11.6C )
4. Circulating fluidized-bed (CFB) reactor ( Figure 11.6D )
FTS is a highly exothermic reaction and is sensitive to the reactor tem-
perature. Localized overheating could cause carbon deposition and produc-
tion of methane at the expense of more valuable liquid products. Thus, it is
critical to efficiently remove the large amount of heat produced. This crite-
rion puts a fixed-bed reactor in a disadvantaged condition compared to a
fluidized-bed reactor or slurry-bed reactor.
For low-temperature process (LTFT), slurry-bed reactors are preferred.
Fluidized-bed reactors are preferred for high-temperature process. Initial
designs of SASOL were based on CFB ( Figure 11.6D ) but present designs
have moved to BFB ( Figure 11.6B ). The switchover from CFB was
prompted by 40% lower cost, more space for heat exchange in reactor, the
lowering of catalyst bulk density due to deposition of carbon in a BFB com-
pared to those in CFB. Additionally, CFB had erosion issues from abrasive
iron carbide catalysts in the narrow section of CFB (Dry, 2002, p. 232).
Multitubular fixed bed ( Figure 11.6A ) and slurry-bed reactors
( Figure 11.6C ) are well suited for LTFT where a large amount of wax is pro-
duced in liquid phase. The syngas enters from the top in the catalysts filled
tubes in the multitubular reactor. The wax trickles down the vertical reactor
tubes ( Figure 11.6 ). In a slurry-bed reactor, the catalyst is suspended in mol-
ten wax product. Syngas bubbles through the slurry bed from the bottom. It
is thus a three-phase reactor. Slurry-bed reactors are about 25% less expen-
sive and has about fourfold reduction rate in catalyst loading and are more
isothermal in nature (Dry, 2002, p. 233) compared to multitubular fixed-bed
reactors.
Furthermore, biomass gasification products contain CO 2 , which is benefi-
cial for the production of liquid products (Reed, 2002, p. 242).